I installed Ubuntu after Windows 7, Then I used to have a menu that let me choose Windows or Ubuntu.
Then for obvious reasons I need re-install windows again, then I lost my dual boot. My computer start windows without ask me nothing.
boot from a live CD, then in a terminal run
sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
this will run a graphical program that will repair your boot loader
Boot the Ubuntu CD again and re-install grub. You can do this graphically with boot-repair
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair
boot-repair
See https://help.ubuntu.com/community/Boot-Repair for details.
If you're on Windows Vista or Windows 7, you can just use EasyBCD. It gives a really beautiful way of managing your boot menu and provides some interesting boot options that you may not easily encounter otherwise.
Alternatively, you can recover your regular grub booting with a utility such as the Super Grub Disk.
There are further options, but those two are the easiest and work just fine.